A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It is known to support the substrate using a substrate support table and/or a vacuum chuck. The known substrate table includes protrusions, for example, nipples or the-like, which support the substrate during use. Preferably, the protrusions are relatively small and/or sharp, so that build up of contamination or dust between the support table and substrate may be prevented. In that case, the substrate may be positioned accurately onto the support table, which is desired for a precise transfer of the pattern onto the substrate during lithography.
For example, EP 0947884 discloses a substrate holder, including a plate having a face which is provided with a matrix arrangement of protrusions for supporting a substrate. The face is also provided with a distributed plurality of apertures extending through the plate. By connecting the apertures in the plate to vacuum generator, the backside of the substrate may be sucked securely against the protrusions.
U.S. Pat. No. 6,257,564 B1 discloses a vacuum chuck, having nipples as support structure. Two types of nipples are used: plain nipples that provide only support, and vacuum nipples that provide support and deliver vacuum to retain the wafer on the chuck. The nipples reduce the contact area between the wafer and the chuck so that contamination of the wafer may be reduced. According to U.S. Pat. No. 6,257,564 B1, warpage of relatively large 300 mm wafers may be avoided.
U.S. Pat. No. 6,664,549 B2 discloses another embodiment of a vacuum chuck, wherein a rear surface of a wafer is held on support pins by suction with a suction chamber.
During use, for example, during the imaging in a lithography method, the substrate may heat up. On the other hand, in case the substrate has been wetted by liquid, for example, when immersion lithography is applied, heat may removed from the substrate via evaporation of the liquid. Such heat flows lead to thermal expansion and/or contraction of the substrate, which may hamper a desired accurate positioning of the substrate. For example, thermal fluctuations of the substrate may cause overlay errors, when the substrate is being subjected to a lithographic process.